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Effects of an epinephrine synthesis inhibitor, SKF64139, on the secretion of luteinizing hormone in ovariectomized female rats
Brain Research, 204 (1981) 231-235 © Elsevier/North-Holland Biomedical Press
231
Effects of an epinephrine synthesis inhibitor, SKF64139, on the secretion of luteinizing hormone in ovariectomized female rats
WILLIAM R. CROWLEY and L. CASS TERRY* Departments of Pharmacology and (L. C.T.) Neurology, University of Tennessee, Centerfor the Health Sciences, Memphis, Tenn. 38163 (U.S.A.)
(Accepted August 7th, 1980) Key words: dopamine --epinephrine -- hypothalamus - - luteinizing hormone - - norepinephrine - -
phenylethanolamine-N-methyltransferase- - SKF64139
In order to test the involvement of central epinephrine systems in luteinizing hormone (LID secretion, the epinephrine synthesis inhibitor, SKF64139, was administered to ovariectomized rats treated with ovarian hormones. This agent significantly decreased resting LH concentrations in ovariectomized, hormonally untreated rats and completely suppressed the stimulatory feedback effects of estrogen plus progesterone. In these animals, hypothalamic concentrations of epinephrine, but not norepinephrine or dopamine, were significantly decreased. It is suggested that brain epinephrine may participate in the neural control of LH secretion in rats. There is abunant evidence that central catecholaminergic systems participate in the neural control over secretion of luteinizing hormone (LH) from the anterior pituitaty gland 21. That norepinephrine (NE) is a stimulatory neurotransmitter for L H release is suggested by the findings that N E receptor blockers and synthesis inhibitors suppress the pulsatile secretion of L H in ovariectomized female ratsS, 2°, block ovulation 11, and also prevent the surge of L H release induced by administration of estradioi or progesterone to ovariectomized, estrogen-primed rats s-l°. Conversely, intracerebroventricular administration of norepinephrine elevates circulating L H titers16,19. To date, however, there have been no attempts to establish whether the neuroendocrine effects of these agents may be due to an action on brain epinephrine systems, in addition to or instead of, an action on norepinephrine. For example, the N E receptor blockers and synthesis inhibitors used previously should disrupt adrenergic, as well as noradrenergic neurotransmission. Moreover, epinephrine is the most potent catecholamine in stimulating L H release when applied intracerebroventricularly 16,19. The purpose of the present experiment was to test the possibility that central epinephrine systems could be involved in the physiologic control of L H secretion. To this end, the effects of a selective epinephrine antagonist, the phenylethanolamine-N-methyltransferase inhibitor SKF6413914,17, on L H secretion were evaluated. Specifically, it was hypothesized that this drug would reduce L H levels in ovariectomized females, potentiate the acute inhibitory feedback effect of estrogen, and blunt the stimulatory effect of an estrogen plus progesterone regime.
232 Sixty day old, Sprague-Dawley female rats (Harlan Industries) were ovariectomized bilaterally and maintained in groups of 5 on a 14 h light: 10 h dark cycle (lights on at 05.000 h) and ad libitum food and water. After 4 weeks recovery, the females received one of three hormone treatments. Ona group was given 50/~g estradiol benzoate (EB) subcutaneously at 11.00 h and was sacrificed at 13.00 h (inhibitory feedback). A second group received 50/zg of EB at 11.00 h on day 0 and 2 mg progesterone 72 h later (stimulatory feedback). These animals were sacrificed 6 h after progesterone. A third group received oil vehicle only at 11.00 h on day 0 and was sacrificed at 13.00 h. For one-half of the animals treated with EB or oil, the epinephrine synthesis inhibitor SKF64139 (50 mg/kg, i.p.), was administered 30 min prior to the hormone injection. The remaining animals in these groups received saline at this time. For the group treated with estrogen plus progesterone, one-half received SKF64139 at 30 rain prior to and again 2.5 h after progesterone, while the remaining animals were given saline vehicle. Following decapitation, trunk blood was collected, plasma separated and stored at--85 °C until LH was measured with N I A M D D radioimmunoassay reagents and procedures. The hypothalamus from each animal was dissected rapidly and frozen with dry ice. These tissues were homogenized in 1 ml of 0.1 N HC104 and centrifuged for 30 min at 10,000 rpm and 4 °C. Catecholamines were measured in aliquots of 60/~1 by a radioenzymatic method 15,1s. The effects of SKF64139 on LH secretion in the 3 hormone groups are presented in Fig. 1. These data were subjected to single factor analysis of variance and Newman-Keuls tests after conversion to natural logarithms, in order to stabilize the variance. Fig. 1 shows that in drug-free animals, administration of EB significantly lowered plasma LH concentrations 2 h later, while the combined EB plus progesterone regime induced a surge in LH secretion. The epinephrine synthesis inhibitor, SKF64139, decreased resting LH levels in oil vehicle-treated females (P < 0.05) and completely P suppressed the LH surge in response,to E B plus progesterone (P < 0.01). The drug also tended to potentiate the inhibitory feedback effects of estradiol, but this effect did not reach statistical significance.
SALINE SKF64139 (50mg/kg)
"~ 600
l
O__~ nr.,=, 200t
VEH
EB
EB+ P
Fig. 1. Effects of SKF64139 on LH secretion in ovariectomizedfemale rats treated with oil vehicle (VEH), estradiot benzoate (EB) or EB plus progesterone (P). *, P < 0.05; **, P < 0.01 vs saline. (n) equals number of observations per group.
233
60I ~ SALINE SKF64139 (50mg/kg)
"' 40V 1 0 0 0 1 ~
V
EB EB+P
EB EB P
6001 1ii
V
EB EB+P
Fig. 2. Effects of SKF64139 on hypothalamic concentrations (ng/g) of norepinephrine (NE),
epinephrine(EPI) and dopamine(DA) in ovariectomizedfemalestreatedwithoil vehicle(V),estradiol benzoate (El}) or EB plus progesterone(EB + P). * P < 0.05 vs saline,based on singlefactoranalysis of variance and Newman-Keulstests. Number of observationssame as Fig. 1. The neurochemical results are shown in Fig. 2. As demonstrated previouslyn, SKF64139 did not alter either NE or dopamine levels in the hypothalamus. However, in oil vehicle and EB plus progesterone-treated animals, epinephrine concentrations were significantly decreased by the drug (Ps < 0.05). In EB-treated rats, SKF64139 produced a small, non-significant decrease in epinephrine concentrations. The present experiment provides preliminary evidence in favor of a stimulatory role of central epinephrine systems in the regulation of LH secretion in female rats. Administration of the epinephrine synthesis inhibitor, SKF64139, to ovariectomized, hormonally untreated rats significantly decreased plasma LH concentrations. Similar results have been obtained previously with adrenergic receptor blockers and dopamine-~-hydroxylase inhibitors, effects that were attributed to central noradrenergic mechanismsS, 2o. SKF64139 also completely blocked the stimulatory effect of progesterone on LH secretion in estrogen-primed animals. Again, the similar findings obtained with catecholamine antagonists in previous studiess-10 have led to the hypothesis that the stimulatory feedback effects of ovarian hormones are mediated by noradrenergic mechanisms. In the present experiment, LH secretion was decreased with unchanged hypothalamic NE concentrations. Rather, decreased hypothalamic epinephrine accompanied the decrease in LH secretion, and this suggests that epinephrine, rather than norepinephrine, may be more important in signalling LH release. Indeed, several recent studies have cast doubt on a necessary role of NE in stimulatory feedback mechanisms; chronic depletion of hypothalamic NE achieved by mechanical or 6-hydroxydopamine-induced lesions of the ascending ventral noradrenergic bundle failed to disrupt estrous cycling or LH surges induced by estrogen and progesteroneZ.8,1L It is interesting to note that central epinephrine systems are reportedly insensitive to 6-hydroxydopamine neurotoxicity7.
234 The notion of a stimulatory action of epinephrine is consistent with the results o f several studies showing that intracerebroventricular administration of epinephrine readily elicits L H release16,19. Moreover, immunocytochemical studies demonstrate putative epinephrine nerve terminals in the periventricular and medial basal hypothalamus ~, areas known to be involved in regulation of gonadotropin secretion. While interference with central adrenergic neurotransmission by SKF64139 appears to be a likely explanation for the present results, peripherally mediated effects cannot be ruled out conclusively at this time. It is unlikely that interference with adrenal epinephrine synthesis is responsible for the antagonism of L H release because adrenalectomy does not prevent estrous cycling or ovulation 4. It is also possible that SKF64139 depressed the sensitivity of the pituitary gland to LH-releasing hormone. SKF64139 is reported to have only weak a-adrenergic blocking actions 14, but may exert some antidopaminergic effects~L Further experiments are in progress in order to confirm a central antiadrenergic site of action for the drug in lowering L H secretion. SKF64139 failed to decrease hypothalamic epinephrine levels significantly in animals treated with estradiol. Because the degree of depletion of catecholamines after a synthesis inhibitor is thought to reflect the turnover of the system 1, it is therefore possible that the estrogen treatment decreased the activity of central adrenergic systems. Further experiments are in progress to determine whether ovarian hormones affect the turnover rates of hypothalamic epinephrine. In summary, the epinephrine synthesis inhibitor, SKF64139, lowered hypothalamic epinephrine without affecting other catecholamines, decreased plasma L H concentrations and prevented ovarian hormone-induced L H surges in ovariectomized female rats. It is suggested that epinephrine may be an excitatory neurotransmitter for L H secretion. The authors thank Mr. D. Smith for expert technical assistance and Ms. B. Chandler for secretarial aid. This research was supported by USPHS Grant RR-05423 to the UTCHS and by N I H Grant HD-13703. Materials for radioimmunoassay of L H were generously supplied by the N I A M D D Rat Pituitary Hormone Distribution Program and Dr. A. F. Parlow. SKF64139 was a kind gift from Smith, Kline and French Laboratories.
1 And6n, N. E., Corrodi, H., Dahlstr6m, A., Fuxe, K. and H6kfelt, T., Effects of tyrosine hydroxylase inhibition on the amine levels of central monoamine neurons, Life Sci., 5 (1966) 561-568. 2 Clifton, D. K. and Sawyer, C. H., LH release and ovulation in the rat following depletion of hypothalamic norepinephrine: chronic vs. acute effects, Neuroendocrinology, 28 (i979) 442-449. 3 Clifton, D. K. and Sawyer, C. H., Positive and negative feedback effects of ovarian steroids on luteinizing hormone release in ovariectomized rats following chronic depletion of hypothalamic norepinephrin¢, Endocrinology, 106 (1980) 1099-t102. 4 Feder, H. H., Brown-Grant, K. and Corker, C. S., Preovulatory progesterone, the adrenal cortex and the critical period for lutein~ng hormone release in rats, J. Endocr., 50 (1971) 29-39. 5 Gnodde, H. P. and Schuiling, G. A., Involvement of catocholaminergic and cholinergic mechanisms in the pulsatile release of LH in the long term ovariect0mized rat, Neuroendocrinology, 20 (1976) 212-223.
235 6 H6kfelt, T., Fuxe, K., Goldstein, M. and Johansson, O., Immunohistochemical evidence for the existence of adrenaline neurons in rat brain, Brain Research, 66 (1974) 235-251. 7 Jonsson, G., Fuxe, K., H6kfelt, T. and Goldstein, M., Resistance of central phenylethanolamineN-methyltransferase containing neurons to 6-hydroxydopamine, Med. Biol., 54 0976) 421-426. 8 Kalra, P. S., Kalra, S. P., Krulich, L., Fawcett, C. P. and McCann, S. M., Involvement of norepinephrine in transmission of the stimulatory influence of progesterone on gonadotropin release, Endocrinology, 90 (1972) 1168-1176. 9 Kalra, P. S. and McCann, S. M., Involvement of catecholamines in feedback mechanisms. In E. Zimmermann,W. H. Gispen, B. H. Marks and D. De Wied (Eds.), Drug Effects on Neuroendocrine Regulation, Progr. Brain Res., Vol. 39, Elsevier, Amsterdam, 1973, pp. 185-198. 10 Kalra, S. P., Chen, C. L. and Clemens, J. A., Effect of norepinephrine synthesis inhibitors and a dopamine agonist on hypothalamic LH-RH, serum gonadotrophin and prolactin levels in gonadal steroid treated rats, Acta endocr. (Kbh.), 89 (1978) 1-9. 11 Kalra, S. P. and McCann, S. M., Effect of drugs modifying catecholamine synthesis on plasma LH and ovulation in the rat, Neuroendocrinology, 15 (1974) 79-91. 12 Lcw, J. W., Baba, Y., Nakamura, S., Engel, J. and Goldstein, M., The antiparkinsonianefficacy and dopaminerphillic properties of ergot alkaloids and of tetrahydroisoquinolines. In E. Usdin, I. J. Kopin, and J. Barchas (Eds.), Catecholamines; Basic and Clinical Frontiers, Pergamon Press, New York, 1979, pp. 1599-1601. 13 Nicholson, G., Greeley, G., Humm, J., Youngblood, W. and Kizer, J.S., Lack of effect of noradrenergic denervation of the hypothalamus and medial preoptic area on the feedback regulation of gonadotrophin secretion and estrous cycle of the rat, Endocrinology, 103 (1978) 559-566. 14 Pendleton, R. G., Kaiser, C. and Gessner, G., Studies on adrenal phenylethanolamine N-methyltransferase ( P N M T ) with S K & F 64139, a selective inhibitor, J. Pharmacol. exp. Ther., 197 (1976) 623-632. 15 Pettier,J. D. and Johnson, G. A., Simultaneous single isotope radioenzymatic assay of plasma norepinephrine, epinephrine, and dopamine, Life Sci., 21 (1977) 625-636. 16 Rubinstein, L. and Sawyer, C. H., Role of catecholamines in stimulating the release of pituitary ovulatory hormone(s) in rats, Endocrinology, 86 (1970) 988-995. 17 Sauter, A. M., Lew, J. Y., Baba, Y. and Goldstein, M., Effect of phenylethanolamineN-methyltransferase and dopamine-fl-hydroxylase inhibition on epinephrine levels in the brain, Life Sci., 21 (1977) 261-266. 18 Sole, M. J. and Hussain, M. N., A simple specific radio-enzymatic assay for the simultaneous measurement of picogram quantities of norepinephrine, epinephrine, and dopamine in plasma and tissues, Biochem. Med., 18 (1977) 301-307. 19 Vijayan, E. and McCann, S. M., Reevaluation of the role of catecholamines in control of gonadotropin and prolactin release, Neuroendocrinology, 25 (1978) 150-165. 20 Weick, R. F., Acute effects of adrenergic receptor blocking drugs and neuroleptic agents on pulsatile discharges of luteinizing hormone in the ovariectomized rat, Neuroendocrinology, 26 (1978) 108-117. 21 Weiner, R. I. and Ganong, W. F., Role of brain monoamines and histamine in regulation of anterior pituitary secretion, Physiol. Rev., 58 (1978) 905-976.
231
Effects of an epinephrine synthesis inhibitor, SKF64139, on the secretion of luteinizing hormone in ovariectomized female rats
WILLIAM R. CROWLEY and L. CASS TERRY* Departments of Pharmacology and (L. C.T.) Neurology, University of Tennessee, Centerfor the Health Sciences, Memphis, Tenn. 38163 (U.S.A.)
(Accepted August 7th, 1980) Key words: dopamine --epinephrine -- hypothalamus - - luteinizing hormone - - norepinephrine - -
phenylethanolamine-N-methyltransferase- - SKF64139
In order to test the involvement of central epinephrine systems in luteinizing hormone (LID secretion, the epinephrine synthesis inhibitor, SKF64139, was administered to ovariectomized rats treated with ovarian hormones. This agent significantly decreased resting LH concentrations in ovariectomized, hormonally untreated rats and completely suppressed the stimulatory feedback effects of estrogen plus progesterone. In these animals, hypothalamic concentrations of epinephrine, but not norepinephrine or dopamine, were significantly decreased. It is suggested that brain epinephrine may participate in the neural control of LH secretion in rats. There is abunant evidence that central catecholaminergic systems participate in the neural control over secretion of luteinizing hormone (LH) from the anterior pituitaty gland 21. That norepinephrine (NE) is a stimulatory neurotransmitter for L H release is suggested by the findings that N E receptor blockers and synthesis inhibitors suppress the pulsatile secretion of L H in ovariectomized female ratsS, 2°, block ovulation 11, and also prevent the surge of L H release induced by administration of estradioi or progesterone to ovariectomized, estrogen-primed rats s-l°. Conversely, intracerebroventricular administration of norepinephrine elevates circulating L H titers16,19. To date, however, there have been no attempts to establish whether the neuroendocrine effects of these agents may be due to an action on brain epinephrine systems, in addition to or instead of, an action on norepinephrine. For example, the N E receptor blockers and synthesis inhibitors used previously should disrupt adrenergic, as well as noradrenergic neurotransmission. Moreover, epinephrine is the most potent catecholamine in stimulating L H release when applied intracerebroventricularly 16,19. The purpose of the present experiment was to test the possibility that central epinephrine systems could be involved in the physiologic control of L H secretion. To this end, the effects of a selective epinephrine antagonist, the phenylethanolamine-N-methyltransferase inhibitor SKF6413914,17, on L H secretion were evaluated. Specifically, it was hypothesized that this drug would reduce L H levels in ovariectomized females, potentiate the acute inhibitory feedback effect of estrogen, and blunt the stimulatory effect of an estrogen plus progesterone regime.
232 Sixty day old, Sprague-Dawley female rats (Harlan Industries) were ovariectomized bilaterally and maintained in groups of 5 on a 14 h light: 10 h dark cycle (lights on at 05.000 h) and ad libitum food and water. After 4 weeks recovery, the females received one of three hormone treatments. Ona group was given 50/~g estradiol benzoate (EB) subcutaneously at 11.00 h and was sacrificed at 13.00 h (inhibitory feedback). A second group received 50/zg of EB at 11.00 h on day 0 and 2 mg progesterone 72 h later (stimulatory feedback). These animals were sacrificed 6 h after progesterone. A third group received oil vehicle only at 11.00 h on day 0 and was sacrificed at 13.00 h. For one-half of the animals treated with EB or oil, the epinephrine synthesis inhibitor SKF64139 (50 mg/kg, i.p.), was administered 30 min prior to the hormone injection. The remaining animals in these groups received saline at this time. For the group treated with estrogen plus progesterone, one-half received SKF64139 at 30 rain prior to and again 2.5 h after progesterone, while the remaining animals were given saline vehicle. Following decapitation, trunk blood was collected, plasma separated and stored at--85 °C until LH was measured with N I A M D D radioimmunoassay reagents and procedures. The hypothalamus from each animal was dissected rapidly and frozen with dry ice. These tissues were homogenized in 1 ml of 0.1 N HC104 and centrifuged for 30 min at 10,000 rpm and 4 °C. Catecholamines were measured in aliquots of 60/~1 by a radioenzymatic method 15,1s. The effects of SKF64139 on LH secretion in the 3 hormone groups are presented in Fig. 1. These data were subjected to single factor analysis of variance and Newman-Keuls tests after conversion to natural logarithms, in order to stabilize the variance. Fig. 1 shows that in drug-free animals, administration of EB significantly lowered plasma LH concentrations 2 h later, while the combined EB plus progesterone regime induced a surge in LH secretion. The epinephrine synthesis inhibitor, SKF64139, decreased resting LH levels in oil vehicle-treated females (P < 0.05) and completely P suppressed the LH surge in response,to E B plus progesterone (P < 0.01). The drug also tended to potentiate the inhibitory feedback effects of estradiol, but this effect did not reach statistical significance.
SALINE SKF64139 (50mg/kg)
"~ 600
l
O__~ nr.,=, 200t
VEH
EB
EB+ P
Fig. 1. Effects of SKF64139 on LH secretion in ovariectomizedfemale rats treated with oil vehicle (VEH), estradiot benzoate (EB) or EB plus progesterone (P). *, P < 0.05; **, P < 0.01 vs saline. (n) equals number of observations per group.
233
60I ~ SALINE SKF64139 (50mg/kg)
"' 40V 1 0 0 0 1 ~
V
EB EB+P
EB EB P
6001 1ii
V
EB EB+P
Fig. 2. Effects of SKF64139 on hypothalamic concentrations (ng/g) of norepinephrine (NE),
epinephrine(EPI) and dopamine(DA) in ovariectomizedfemalestreatedwithoil vehicle(V),estradiol benzoate (El}) or EB plus progesterone(EB + P). * P < 0.05 vs saline,based on singlefactoranalysis of variance and Newman-Keulstests. Number of observationssame as Fig. 1. The neurochemical results are shown in Fig. 2. As demonstrated previouslyn, SKF64139 did not alter either NE or dopamine levels in the hypothalamus. However, in oil vehicle and EB plus progesterone-treated animals, epinephrine concentrations were significantly decreased by the drug (Ps < 0.05). In EB-treated rats, SKF64139 produced a small, non-significant decrease in epinephrine concentrations. The present experiment provides preliminary evidence in favor of a stimulatory role of central epinephrine systems in the regulation of LH secretion in female rats. Administration of the epinephrine synthesis inhibitor, SKF64139, to ovariectomized, hormonally untreated rats significantly decreased plasma LH concentrations. Similar results have been obtained previously with adrenergic receptor blockers and dopamine-~-hydroxylase inhibitors, effects that were attributed to central noradrenergic mechanismsS, 2o. SKF64139 also completely blocked the stimulatory effect of progesterone on LH secretion in estrogen-primed animals. Again, the similar findings obtained with catecholamine antagonists in previous studiess-10 have led to the hypothesis that the stimulatory feedback effects of ovarian hormones are mediated by noradrenergic mechanisms. In the present experiment, LH secretion was decreased with unchanged hypothalamic NE concentrations. Rather, decreased hypothalamic epinephrine accompanied the decrease in LH secretion, and this suggests that epinephrine, rather than norepinephrine, may be more important in signalling LH release. Indeed, several recent studies have cast doubt on a necessary role of NE in stimulatory feedback mechanisms; chronic depletion of hypothalamic NE achieved by mechanical or 6-hydroxydopamine-induced lesions of the ascending ventral noradrenergic bundle failed to disrupt estrous cycling or LH surges induced by estrogen and progesteroneZ.8,1L It is interesting to note that central epinephrine systems are reportedly insensitive to 6-hydroxydopamine neurotoxicity7.
234 The notion of a stimulatory action of epinephrine is consistent with the results o f several studies showing that intracerebroventricular administration of epinephrine readily elicits L H release16,19. Moreover, immunocytochemical studies demonstrate putative epinephrine nerve terminals in the periventricular and medial basal hypothalamus ~, areas known to be involved in regulation of gonadotropin secretion. While interference with central adrenergic neurotransmission by SKF64139 appears to be a likely explanation for the present results, peripherally mediated effects cannot be ruled out conclusively at this time. It is unlikely that interference with adrenal epinephrine synthesis is responsible for the antagonism of L H release because adrenalectomy does not prevent estrous cycling or ovulation 4. It is also possible that SKF64139 depressed the sensitivity of the pituitary gland to LH-releasing hormone. SKF64139 is reported to have only weak a-adrenergic blocking actions 14, but may exert some antidopaminergic effects~L Further experiments are in progress in order to confirm a central antiadrenergic site of action for the drug in lowering L H secretion. SKF64139 failed to decrease hypothalamic epinephrine levels significantly in animals treated with estradiol. Because the degree of depletion of catecholamines after a synthesis inhibitor is thought to reflect the turnover of the system 1, it is therefore possible that the estrogen treatment decreased the activity of central adrenergic systems. Further experiments are in progress to determine whether ovarian hormones affect the turnover rates of hypothalamic epinephrine. In summary, the epinephrine synthesis inhibitor, SKF64139, lowered hypothalamic epinephrine without affecting other catecholamines, decreased plasma L H concentrations and prevented ovarian hormone-induced L H surges in ovariectomized female rats. It is suggested that epinephrine may be an excitatory neurotransmitter for L H secretion. The authors thank Mr. D. Smith for expert technical assistance and Ms. B. Chandler for secretarial aid. This research was supported by USPHS Grant RR-05423 to the UTCHS and by N I H Grant HD-13703. Materials for radioimmunoassay of L H were generously supplied by the N I A M D D Rat Pituitary Hormone Distribution Program and Dr. A. F. Parlow. SKF64139 was a kind gift from Smith, Kline and French Laboratories.
1 And6n, N. E., Corrodi, H., Dahlstr6m, A., Fuxe, K. and H6kfelt, T., Effects of tyrosine hydroxylase inhibition on the amine levels of central monoamine neurons, Life Sci., 5 (1966) 561-568. 2 Clifton, D. K. and Sawyer, C. H., LH release and ovulation in the rat following depletion of hypothalamic norepinephrine: chronic vs. acute effects, Neuroendocrinology, 28 (i979) 442-449. 3 Clifton, D. K. and Sawyer, C. H., Positive and negative feedback effects of ovarian steroids on luteinizing hormone release in ovariectomized rats following chronic depletion of hypothalamic norepinephrin¢, Endocrinology, 106 (1980) 1099-t102. 4 Feder, H. H., Brown-Grant, K. and Corker, C. S., Preovulatory progesterone, the adrenal cortex and the critical period for lutein~ng hormone release in rats, J. Endocr., 50 (1971) 29-39. 5 Gnodde, H. P. and Schuiling, G. A., Involvement of catocholaminergic and cholinergic mechanisms in the pulsatile release of LH in the long term ovariect0mized rat, Neuroendocrinology, 20 (1976) 212-223.
235 6 H6kfelt, T., Fuxe, K., Goldstein, M. and Johansson, O., Immunohistochemical evidence for the existence of adrenaline neurons in rat brain, Brain Research, 66 (1974) 235-251. 7 Jonsson, G., Fuxe, K., H6kfelt, T. and Goldstein, M., Resistance of central phenylethanolamineN-methyltransferase containing neurons to 6-hydroxydopamine, Med. Biol., 54 0976) 421-426. 8 Kalra, P. S., Kalra, S. P., Krulich, L., Fawcett, C. P. and McCann, S. M., Involvement of norepinephrine in transmission of the stimulatory influence of progesterone on gonadotropin release, Endocrinology, 90 (1972) 1168-1176. 9 Kalra, P. S. and McCann, S. M., Involvement of catecholamines in feedback mechanisms. In E. Zimmermann,W. H. Gispen, B. H. Marks and D. De Wied (Eds.), Drug Effects on Neuroendocrine Regulation, Progr. Brain Res., Vol. 39, Elsevier, Amsterdam, 1973, pp. 185-198. 10 Kalra, S. P., Chen, C. L. and Clemens, J. A., Effect of norepinephrine synthesis inhibitors and a dopamine agonist on hypothalamic LH-RH, serum gonadotrophin and prolactin levels in gonadal steroid treated rats, Acta endocr. (Kbh.), 89 (1978) 1-9. 11 Kalra, S. P. and McCann, S. M., Effect of drugs modifying catecholamine synthesis on plasma LH and ovulation in the rat, Neuroendocrinology, 15 (1974) 79-91. 12 Lcw, J. W., Baba, Y., Nakamura, S., Engel, J. and Goldstein, M., The antiparkinsonianefficacy and dopaminerphillic properties of ergot alkaloids and of tetrahydroisoquinolines. In E. Usdin, I. J. Kopin, and J. Barchas (Eds.), Catecholamines; Basic and Clinical Frontiers, Pergamon Press, New York, 1979, pp. 1599-1601. 13 Nicholson, G., Greeley, G., Humm, J., Youngblood, W. and Kizer, J.S., Lack of effect of noradrenergic denervation of the hypothalamus and medial preoptic area on the feedback regulation of gonadotrophin secretion and estrous cycle of the rat, Endocrinology, 103 (1978) 559-566. 14 Pendleton, R. G., Kaiser, C. and Gessner, G., Studies on adrenal phenylethanolamine N-methyltransferase ( P N M T ) with S K & F 64139, a selective inhibitor, J. Pharmacol. exp. Ther., 197 (1976) 623-632. 15 Pettier,J. D. and Johnson, G. A., Simultaneous single isotope radioenzymatic assay of plasma norepinephrine, epinephrine, and dopamine, Life Sci., 21 (1977) 625-636. 16 Rubinstein, L. and Sawyer, C. H., Role of catecholamines in stimulating the release of pituitary ovulatory hormone(s) in rats, Endocrinology, 86 (1970) 988-995. 17 Sauter, A. M., Lew, J. Y., Baba, Y. and Goldstein, M., Effect of phenylethanolamineN-methyltransferase and dopamine-fl-hydroxylase inhibition on epinephrine levels in the brain, Life Sci., 21 (1977) 261-266. 18 Sole, M. J. and Hussain, M. N., A simple specific radio-enzymatic assay for the simultaneous measurement of picogram quantities of norepinephrine, epinephrine, and dopamine in plasma and tissues, Biochem. Med., 18 (1977) 301-307. 19 Vijayan, E. and McCann, S. M., Reevaluation of the role of catecholamines in control of gonadotropin and prolactin release, Neuroendocrinology, 25 (1978) 150-165. 20 Weick, R. F., Acute effects of adrenergic receptor blocking drugs and neuroleptic agents on pulsatile discharges of luteinizing hormone in the ovariectomized rat, Neuroendocrinology, 26 (1978) 108-117. 21 Weiner, R. I. and Ganong, W. F., Role of brain monoamines and histamine in regulation of anterior pituitary secretion, Physiol. Rev., 58 (1978) 905-976.